In a personal statement, she said she was making “good progress” in her treatment but was “not out of the woods yet” and had “good days and bad days”.
It is understood that she was particularly focused on supporting her children during their public outing at Trooping, as well as the King.
She has not been seen on public duty so far this year, having major abdominal surgery in January and in March releasing a personal video telling the world she was having preventative chemotherapy following the discovery of cancer.
At the time of the update, on March 22, the palace said the Princess would return to official duties only when she was “cleared to do so by her medical team”.
“She may be keen to attend events as and when she feels able to, in line with medical advice,” added a source then. “Any initial public events will not necessarily signal a return to a regular public schedule.”
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It was the first time the princess has been photographed with the wider royal family since Christmas Day, on the annual walk to Sandringham church.
On January 17, Kensington Palace announced the princess was undergoing abdominal surgery, in hospital for up to two weeks and out of the public eye until at least Easter.
She was seen with Prince George, Princess Charlotte and Prince Louis in a Mother’s Day photograph, taken by Prince William and later revealed to have been edited.
In March, the Sun published a video of the prince and princess out at the Windsor Farm Shop.
Shortly afterwards, the princess filmed her own video explaining that she had experienced complications following her surgery.
Post-operative tests had shown cancer had been present, she said, and she had begun a course of preventative chemotherapy.
“We hope that you will understand that, as a family, we now need some time, space and privacy while I complete my treatment,” she said then.
“My work has always brought me a deep sense of joy and I look forward to being back when I am able, but for now I must focus on making a full recovery.”
The princess has since been kept abreast of developments in her Early Years project, and wrote a letter to the Irish Guards, but has kept entirely out of the public eye.
In a statement released on Friday, she said: “I’m looking forward to attending The King’s Birthday Parade this weekend with my family and hope to join a few public engagements over the summer, but equally knowing I am not out of the woods yet.”
The Telegraph, London